In addition to endoscopy and conventional X-ray colonography, virtual CT colonography or CT colonoscopy derived from it is becoming significantly important. Bowel cancer represents a frequent cause of death in western countries. One good indicator of this illness is polyps which are formed a relatively long time before the outbreak of bowel cancer in the bowel. In many cases, these develop over a number of years to form malign carcinomas.
One effective measure against bowel cancer is thus early identification of polyps. In the past, endoscopic methods were used for this purpose. Recently, the technique of virtual colonography has become important, in which the bowel is scanned by means of a CT scanner (CT) or a magnetic resonance imaging scanner (MRI) and the images obtained during this process are then analyzed at medical workstations.
In the case of virtual colonography, the bowel must be cleaned by way of suitable medication before the examination, in order to make it possible to identify structures which indicate polyps or other conspicuous features on the bowel wall in the later images. Patients find this to be unpleasant, and it therefore frequently represents an obstacle to the examination.
The technique of so-called virtual bowel cleaning is also known, in addition to this actual bowel cleaning. In the case of this virtual bowel cleaning, the volume data record of the bowel as recorded using a CT scanner is subjected to post-processing in which voxels which relate to the bowel content are identified in the volume data record and are eliminated by digital subtraction from the volume data record. The delineation of the bowel and bowel content required for this purpose is made easier by the introduction of contrast agents. These contrast agents are either added to the food which the patient consumes in a time period before the examination, or are given to the patient intravenously immediately before the examination.
The first situation results in contrasting of the bowel content, while the second case results in contrasting of the bowel with respect to the bowel content. In addition, before the examination, the bowel is expanded by pumping in surrounding air or CO2 gas in order to allow it to be displayed better. Those voxels in the volume data record which relate to the bowel content are in this case identified on the basis of the CT attenuation volumes of these voxels, or on the basis of changes in the CT attenuation values between mutually adjacent areas. Both are known, for example, from U.S. Pat. No. 6,331,116 B1, which discloses a method of this generic type for virtual bowel cleaning.
Despite all the known techniques, residual stool in the bowel always results in problems relating to delineation from the tissue, in particular from the bowel wall. One conventional strategy is to carry out the examination first of all lying on the back and then once again lying on the stomach, in the hope that the location of the bowel contents would change during the process, in this way identifying the contents as stool. However, this is not always successful.